1. Field of the Invention
The present invention relates generally to a filter using a film bulk acoustic resonator (hereinafter, it will be referred to as “FBAR”), a duplexer using the filter, and fabrication methods thereof. More particularly, the present invention relates to a filter which is fabricated by combining inductors for adjusting resonance characteristics with FBARs as a single chip in a serial and parallel manner, a duplexer using the filter, and fabrication methods thereof.
2. Description of the Related Art
In recent years, demand for mobile communication devices such as cellular phones is rapidly increasing, so that demand for a small-sized and light-weight filter and a duplexer used in such mobile communication devices is also increasing. In the meantime, the FBAR is known to be suitable for a small and light-weight filter. The FBAR has advantages in that it allows mass production at a minimum cost and allows the minimum size implementation. In addition, it has advantages in that it may implement a high quality factor Q which is a main characteristic of the filter, it may be utilized in a microwave band, and in particular it may also be utilized in a personal communication system (PCS) band and a digital cordless system (DCS) band.
In general, the FBAR device includes a resonant portion on a substrate where a bottom electrode, a piezoelectric layer, and a top electrode are sequentially stacked. The operating principle of the FBAR is as follows. Electrical energy is applied to the electrode to induce a time-varying magnetic field within the piezoelectric layer, and the magnetic field induces a bulk acoustic wave in the same direction as the vibration direction of the resonant portion within the piezoelectric layer to thereby generate a resonance.
A ladder type filter is one type of filter that uses the FBAR device. The ladder type filter is a band pass filter, wherein a plurality of FBAR devices is combined in a serial and parallel manner to adjust resonance characteristics of each device to thereby pass only signals within a predetermined frequency band.
FIG. 1 is a block diagram illustrating a ladder type filter where a plurality of thin film resonators (TFRs) is combined in a serial and parallel manner as disclosed in U.S. Pat. No. 6,377,136. Referring to FIG. 1, the filter includes a plurality of serial TFRs S1, S2, . . . , SN, and a plurality of parallel TFRs P1, P2, . . . , Pn. Each of the serial TFRs 11, 12, . . . , N is serially connected to each other between an input port and an output port. Each of the parallel TFRs 21, 22, . . . , n connects the ground to each node between two adjacent serial TFRs 11, 12, . . . , N.
FIG. 2 is a graph illustrating impedance characteristics of serial and parallel TFRs included in the ladder type filter. Referring to FIG. 2, f1 and f2 of the impedance characteristic graph 30 of the serial TFR indicate antiresonance frequency and resonance frequency, respectively, and f3 and f4 of the impedance characteristic graph 40 of the parallel TFR indicate antiresonance frequency and resonance frequency, respectively. When the frequency characteristic of the parallel TFR or the serial TFR is adjusted so as to match the antiresonance frequency f3 of the parallel TFR with the resonance frequency f2 of the serial TFR, only signals within the frequency band ranging from f1 to f4 are passed, which is the operation of the band pass filter. In this case, the resonance frequency of the band pass filter is f2 which is equal to f3.
In order to adjust the frequency characteristic of the serial TFR or the parallel TFR in the filter shown in FIG. 1, thickness and material for electrode, piezoelectric layer or the like which compose each of the TFRs should be fabricated to be different from one another. As a result, there occurs a difficulty in having fabrication processes different from one another per each of TFRs.
Alternatively, the frequency characteristic of the band pass filter may be adjusted by properly combining the TFRs having uniform frequency characteristics and connecting devices such as inductors to the parallel TFR. However, the connection of an external device such as inductor causes the device volume to be increased. As a result, it is difficult to apply it to a small-sized communication apparatus such as a cellular phone.
Such a filter may be employed in a device such as a duplexer. The duplexer is a device for transceiving signals through one antenna, and has a structure including a transmitting port filter, a receiving port filter, and a filter isolating portion for preventing signal interference between the filters. The transmitting port filter filters only signals to be transmitted outward through the antenna, and the receiving port filter filters only signals to be received from outside the duplexer. The filter isolating portion may be implemented as a phase shifter which serves to prevent interference between the filters by having a phase difference of 90° between the frequency of the transmitting signal and that of the receiving signal. The phase shifter may be implemented typically using a capacitor and an inductor.
The duplexer also uses the filter, which in turn causes the size of the duplexer to be increased in response to the increased size of the fabricated filter. In addition, when the filter fabrication process becomes complicated, it becomes more difficult to fabricate the duplexer.